Turboprop aircraft engines include a propeller and a gas turbine engine. At least one stage of the gas turbine engine acts through a gear reduction system to drive the propeller.
The thrust produced by a turboprop engine is a function of several parameters, including, the rotational speed of the propeller and the angular alignment, or pitch, of the generally flat propeller blades relative to the direction of movement of the aircraft. For example, if the plane of each generally flat propeller blade is aligned nearly parallel to the direction of movement of the aircraft, the propeller will produce little thrust and will create little drag on the aircraft. This blade alignment is referred to as a neutral pitch or a feather pitch. If the plane of each propeller blade is more nearly perpendicular to the direction of movement of the aircraft, the propeller also will produce little thrust but will create a greater amount of drag on the aircraft. This latter position is defined as a fine or flat pitch. The blade pitch will be between these two extremes during most flight conditions. The adjustability of the pitch enables a careful balancing of the forces exerted on the propeller blades by the engine with the forces exerted on the propeller blades by the air through which the turboprop engine is moving.
Most turboprop engines include a complex hydraulic system that senses speed and power lever position, and adjusts the pitch of the propeller blades accordingly to prevent either an underspeed or an overspeed condition. More particularly, a flow of hydraulic fluid, such as engine oil, is created by a governor pump, and is urged through a constant speed governor and toward the propeller pitch adjustment mechanism. The specific operation of the constant speed governor will be explained in greater detail below. However, it should be noted that the governor pump, the constant speed governor and in some applications the overspeed governor are operated through a gear reduction mechanism that is driven by the power turbine of the turboprop engine. The pressure of hydraulic fluid to the propeller pitch adjustment mechanism varies in accordance with the rotational speed of the engine and the internal controls of the constant speed governor. Generally, an underspeed condition will change the hydraulic pressure between the constant speed governor and the propeller pitch adjustment mechanism and will cause the propeller blades to move toward a fine pitch. Conversely, an overspeed condition will cause an opposite change in the hydraulic pressure and will urge the propeller blades toward a coarser pitch.
The hydraulic system which determines the pitch of the propeller blades also includes a lock pitch solenoid which is selectively operable to lock the propeller blades into the then existing pitch. More particularly, the lock pitch solenoid may include a mechanical member which interrupts the flow of hydraulic fluid between the governor and the propeller adjustment mechanism. This interruption will prevent the hydraulic pressure from either increasing or decreasing. Consequently, the propellers will be locked at a constant pitch until some change in the system is put into effect. The lock pitch solenoid typically is activated by a movement of the power lever that would be made during an approach for a landing. Under these circumstances, the lock pitch solenoid is effective in preventing the propellers from moving to a very fine pitch or to a reverse pitch.
The prior art propeller pitch adjustment mechanism also is provided with a means for producing a feather or neutral pitch as a fail-safe in response to a sensed failure in the turbine or the gear reduction mechanism.
The prior art turboprop engines perform well under most conditions. Recently, however, certain engine failures have been observed that were not properly addressed by the existing fail-safe systems. After an investigation of these failures, it was determined that the existing fail-safe controls were too slow in addressing a failure in the governor pump, in the constant speed governor or the complex gear reduction mechanism which drives the governor pump, the constant speed governor and in some applications the overspeed governor. More particularly, it was found that during the short period of time following the above described failure, a residual pressure will continue to develop in the hydraulic system which operates the propeller pitch adjustment mechanism, as a result of the gear coastdown. This residual pressure in a very short time is capable of moving the propeller blades toward fine pitch. The failure causes the internal controls of the governor to move into positions which reflect an underspeed condition. The residual pressure combined with the sensed underspeed condition could cause the propeller pitch adjustment mechanism to move the propeller blades rapidly into a very fine pitch or flat pitch condition, even though the aircraft operating conditions did not warrant such an adjustment. Thus, an overspeed condition would be created. Obviously, this incorrect adjustment to a fine pitch would imbalance the forces exerted on the propeller blades significantly because of the drag inherent with the fine pitch alignment. In some situations the propeller would survive this condition long enough for the prior art fail-safe mechanism to sense the system failure and move the propeller blades into the feather or neutral pitch condition. However, in other instances, the imbalance created by the sudden and unjustified movement of the propeller blades into a very fine or flat pitch would cause catastrophic damage to the turboprop engine, as a consequence of the overstress encountered by the engine-propeller rotating parts.
In view of the above, it is an object of the subject invention to provide an apparatus that will properly control the propeller pitch adjustment mechanism in response to a failure in the governor or the gears driving the governor.
It is another object of the subject invention to provide an apparatus for rapidly sensing a failure in the governor or the apparatus which drives the governor, and which then will appropriately control the propeller pitch adjustment mechanism.
It is an additional object of the subject invention to provide an apparatus for preventing the propeller from moving into a fine pitch in response to a mechanical failure in the turboprop engine.
It is a further object of the subject invention to provide a system that prevents unwarranted overspeed pitch adjustments and that can be readily incorporated into existing turboprop engines.